If you get nervous the second you step into your doctor's office, your heart might start pounding.

"The body essentially reacts in a fight-or-flight manner, increasing the heart rate and getting ready to make a move," says Dr. Kaplan.

Experts call the resulting increase in blood pressure "white coat hypertension," which can cause your reading to spike by as much as 15 points, suggests a review published inHypertension.

Taking deep breaths in through your nose and out through your mouth can help you calm down, which will slow your heart rate and bring your blood pressure back down to normal, says Dr. Kaplan.

MORE:How to De-Stress In Under 2 Minutes

Waiting until the end of your appointment to have your BP measured might help, too. "By then, you'll hopefully have all of your questions answered, and you'll be less anxious," he says.

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Cause #2: You rushed to your appointment.

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It’s happened to all of us: The only available parking spot is a block away—and you have 5 minutes to make it to your doctor’s office on the building’s third floor. That’s bad news for your blood pressure, and not just because of the effect of the mental stress of being late.

When you walk fast or run, your heart rate increases as it pumps more blood to power your muscles. More blood pumping through your vascular system means more pressure on the walls of your blood vessels.

That causes your blood pressure to temporarily rise, says Dr. Kaplan.

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If your doctor takes your blood pressure before your heart rate has had a chance to return to normal—which can take up to 30 minutes, depending on how fast your heart was beating—the reading will probably be higher than you expect.

So instead of having your BP checked at the beginning of your appointment, ask your doctor if he can wait until the end.

"By then, your body's physiology will have come back to a normal resting state," Dr. Kaplan says. And your BP reading will be more accurate. (Discover how reversing chronic inflammation can lower your blood pressure, banish joint pain, halt heart disease, and so much more! Try free for 21 days.)

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Cause #3: You have to pee—badly.

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When your bladder is full, your body signals the release of stress hormones like adrenaline, which activate your fight-or-flight response. That causes your blood vessels to constrict, which can raise your blood pressure by as much as 15 points, says New Providence, New Jersey cardiologist Steve Sheris, MD.

As you might've guessed, the fix is pretty easy. If you get to the doctor's office and you have to pee, don't hold it in. Go, and then let the doc take your BP, Dr. Sheris says. Your BP should go back to normal in three or four minutes.

MORE:How To Stop Getting Up At Night To Pee

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Cause #4: You drank coffee or an energy drink.

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The culprit here is the caffeine, though experts aren't sure exactly how it sends your blood pressure skyrocketing. Two possibilities? It might signal to your body to produce more adrenaline, which speeds up your heart rate. It could also cause your blood vessels to constrict, according to the Mayo Clinic.

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Caffeine seems to affect blood pressure more in people who generally don’t drink coffee than those who are already used to the stuff, says Dr. Kaplan. Regular coffee drinkers could have a spike of up to 5 mm Hg. And if you don't normally drink it, your BP could jump by as much as 15 mm Hg.

So if you don't normally guzzle double espressos, the day of your annual physical isn't the time to start. But even if you drink coffee or energy drinks daily, don’t walk into your appointment with the stuff in hand. Most experts agree that patients should steer clear of caffeine for at least 30 minutes before having their BP taken, Dr. Kaplan says.

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Cause #5: You sat with your legs crossed.

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Sitting with your legs crossed compresses the veins in your legs, which can cause blood to pool up down there. To compensate and make sure enough blood makes it to other important parts of your body—like your chest—your heart starts pumping more blood, says Dr. Sheris. And that sends your blood pressure up, sometimes by as much as 8 mm Hg.

For the most accurate BP reading, you should sit in a comfortable, upright position. Your feet should be flat on the floor and your elbows should rest on the armrest of the chair, Dr. Sheris says. The position is standard protocol, so if you aren't sitting right, your doctor should let you know. (If you're at a self-serve BP reading place like a pharmacy, and you’re not sure that you’re sitting correctly, ask the pharmacist for help.)

MORE:5 Ways To Get The Most Accurate Blood Pressure Reading

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Cause #6: The exam room is freezing.

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When it's chilly, the blood vessels near the surface of your skin constrict to send more blood toward your core. This helps keep your vital organs warm, but it can also drive up your BP by as much as 20 mm Hg, says Dr. Sheris. When your blood vessels are narrower, the blood flowing through them exerts more pressure.

Since you can't exactly crank the heat in the doctor’s building—or keep your coat on when he's taking your BP—the best you can do is make sure you’re not too cold beforehand. If the room seems weirdly chilly, you could leave your coat on until right before the reading. But if you’re always freezing no matter what, mention it to your doctor—she might recommend that you try taking your BP at home.

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So should you worry about one high blood pressure reading?

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If you get one strange reading, don't worry too much. Your doctor should take at least two BP readings, at least one minute apart, and average the two readings, recommends the American Heart Association. But if your BP seems higher than usual, and one of the above factors was in play, mention it to your doctor. He can decide if you should wait longer before taking the second reading.

"It's the average of many BP readings that matters," says Dr. Kaplan. "You need to have elevated readings on multiple days to have the official diagnosis of hypertension."

If the number continues to measure over 140/90 (which is the cutoff for high blood pressure), you and your doctor can talk about lifestyle changes that can help bring them down—like exercising, eating right, cutting back on salt and alcohol, and quitting smoking—as well as any possible meds like acetaminophen, antidepressants, NSAIDs, or corticosteroids.